A number of medical conditions may be treated using ablative techniques or devices. Ablative therapy generally involves the killing of abnormal tissue at an area of interest, thereby resulting in an efficacious treatment for a medical condition. For example, atrial fibrillation may be treatable by ablation of the abnormal tissue within the left atrium and/or the pulmonary vein.
Atrial fibrillation is a serious medical condition that is the result of abnormal electrical activity within the heart. This abnormal activity may occur at regions of the heart including the sino-atrial (SA) node, the atriovenricular (AV) node, or within other areas of cardiac tissue. Moreover, atrial fibrillation may be caused by abnormal activity within one or more isolated focal centers within the heart. It is believed that these foci can originate from within the pulmonary vein, particularly the superior pulmonary veins.
Ablation catheters have been used in minimally invasive techniques to ablate target tissue, e.g., foci having abnormal electrical activity. The techniques typically are characterized by application of energy to create lesions at the foci or other areas possessing abnormal electrical activity. Ablation catheters can also be used to create lesions at the heart to block electrical signals or to alter a travel path of electrical signals at the heart.
Some ablation devices utilize radio frequency (RF) energy for ablation, including the device disclosed in U.S. Pat. No. 6,024,740 to Lesh et al. The RF energy devices may be used to ablate an area of interest with heat. The use of RF energy for ablation may, however, lead to untoward healing responses such as collagen build up at the area of interest after treatment. In some cases, RF ablation may create lesions that cause occlusion of the coronary sinus in post procedure healing. A need, therefore, exists for ablative devices and methods that include improved healing responses.
An alternative treatment strategy has been developed that uses cooling energy for ablation. This method, termed cryoplasty or cryotherapy, may be used to cool or otherwise freeze a portion of target tissue to ablate the target tissue. For example, cryoplasty may be used to cool or freeze and simultaneously dilate a lesion within a blood vessel that might otherwise lead to restenosis or recoil. Cryotherapy may also be used to create lesions at a heart to treat atrial fibrillation. However, creating lesions in a heart using cryotherapy poses a challenge in that it may be difficult to deliver sufficient cooling to create a transmural (i.e., a through thickness) lesion. In addition, blood delivered to and from the heart constantly provides heat to a target site at the heart, thereby counteracting against the cooling being delivered by the cryotherapy, and limiting the amount of cooling that can be delivered to the target site. This in turn, further prevents a transmural lesion, or lesion of a desired size or characteristic, from being created at the target tissue.
Thus, there is currently a need for an improved device and method to perform ablation therapy.